Polishing large cylindrical vessels or tanks with closed ends

ABSTRACT

A method and apparatus for electropolishing or chemical polishing the entire inner surface or portion of a large cylindrical vessel having at least one closed end. The vessel is supported for rotation with its longitudinal axis in an inclined position and its lower end provided with axial thrust support. The polishing liquid is introduced into the vessel in a quantity sufficient to attain a surface level as high as the center point of the lower closed end. The vessel is then rotated about its inclined axis and the entire inner surface of the lower end is polished. The vessel is then repositioned with the unpolished end being lower than the previously polished end and supported in the same manner. The level of the polishing liquid is then adjusted so that the remaining unpolished inner surface will be wetted and polished upon the rotation of the vessel about its inclined longitudinal axis. The vessel is then so rotated while polishing is completed.

United States Patent n91 Jumer POLISHING LARGE CYLINDRICAL VESSELS OR TANKS WITH CLOSED ENDS John F. Jumer. [6 W. l3l Timber Trails Drive, Elmhurst Ill. 60l26 [221 Filed: Dec. 13. 1973 21 Appll NQ;424,|90

[76] Inventor:

[45] Nov. 11, 1975 Prinmr Etaminer-F. C. Edmundson Attorney, Agent, or Firm-Lockwood, Dewey, Zickert & Alex [57] ABSTRACT A method and apparatus for electropolishing or chemical polishing the entire inner surface or portion of a large cylindrical vessel having at least one closed end The vessel is supported for rotation with its longitudinal axis in an inclined position and its lower end pro vided with axial thrust support. The polishing liquid is introduced into the vessel in a quantity sufficient to attain a surface level as high as the center point of the lower closed end. The vessel is then rotated about its inclined axis and the entire inner surface of the lower end is polished. The vessel is then repositioned with the unpolished end being lower than the previously polished end and supported in the same manner The level of the polishing liquid is then adjusted so that the remaining unpolished inner surface will be wetted and polished upon the rotation of the vessel about its inclined longitudinal axis. The vessel is then so rotated while polishing is completed.

it) Claims. 6 Drawing Figures U.S. Patent Nov. 11, 1975 Sheet 1 of2 3,919,061

US. Patent Nov. 11, 1975 Sheet2 of2 3,919,061

POLISHING LARGE CYLINDRICAL VESSELS OR TANKS WITH CLOSED ENDS This invention relates, generally, to innovations and improvements in the chemical or electropolishing of either substantially the entire inner surfaces, or a desired portion thereof, of largecylindrical vessels of the type having at least one closed end which end or ends have been welded to the cylindrical body prior to their being polished. Some vessels may have an open end to accommodate a removable head or cover.

In many industries, such as those processing materialsfor human use or consumption, such as the pharmaceutical, food or beverage industries, sterile conditions and a passive surface are required. In order to facilitate thorough cleaning and sterilization, vessels with polished interior surfaces are desired. In these and other situations it is also desirable toprovide vessels with polished interior surfaces in order to promote high release properties, resistance to corrosion, facilitate cleaning, or for other reasons. While mechanical methods are often used, at times it is more desirable to polish the interior surface of such a vessel by chemical polishing or electropolishing methods.

The art of chemical polishing and electropolishing of metal objects is well developed. For example, in my prior US. Pat. No. 2,861,931, dated Nov. 25, 1958, I have disclosed a method andapparatus for'electropolishing the interior surfaces of large vessels. In my prior US. Pat. Nos. 2,705,191 dated Mar. 29, 1955, and US. Pat. No. 3,041,227 datedJune 26, 1962, I have disclosed improved compositions and methods for chemically polishing the surfaces of aluminum objects. In my prior US. Pat. No. 3,682,799 dated Aug. 8, 1972, I have disclosed methods'of fabricating and electropolishing the interiors of cylindrical vessels with domed ends. In my US. Pat. No. 2,861,937 I disclosed a method of polishing the inner surface of such a cylindrical vessel having substantially closed ends whereby it was necessary that the vessel be filled to at least half of its capacity with the polishing bath. In my US. Pat. No. 3,682,799, I disclosed a method whereby such a vessel would be polished with the use of considerably less polishing bath; but in that method it is necessary that the ends be separately polished prior to their being welded to the cylindrical body. While both of these prior methods are practical and provide benefits previously unknown, occasions may arise when it is preferred to polish the end or ends after they have been attached to the cylindrical body. If, in such a instance, it is desired to polish substantially all of the inner surface of the vessel including its ends, it is necessary to use a volume of polishing bath at least equal to one-half the capacity of the vessel as taught in my US. Pat. No. 2,861,937. When such a vessel is large, say of a capacity of several thousand gallons or more, the large volume of liquid requiredis undesirable and may pose problems in respect to cost, weight, availability, excessive stresses on the vessel or its supports, handling and storage, and objectionably larg'e electrical energy requirements.

Accordingly, the object of my present invention, generally stated, is to provide an improved method for chemical or electropolishing substantially the entire inner surfaces of prefabricated large cylindrical vessels having substantially closed ends using substantially smaller volumes of polishing baths than heretofore required.

Another object of my invention is to provide an improved method of chemical or electropolishing the interior surfaces or prefabricated larger cylindrical vessels having substantially closed ends whereby one end and a substantial portion of the cylindrical body can be polished in one operation and the entire inner surface in two operations.

Another object of my invention is to provide an improved method of chemical or electropolishing the interior surfaces of prefabricated large cylindrical vessels having substantially closed ends whereby the required quantity of liquid for chemical or electropolishing can be minimized by polishing while the vessel is inclined and rotated about its longitudinal axis.

Another object of my invention is to provide an improved method of chemcially or electropolishing the inner surface of one end and up to all of the cylindrical body of a cylindrical vessel in one operation.

Another object of my invention is to provide an improved method for fabricating and chemically polishing or electropolishing the interior surface of large cylindrical vessels having substantially closed do'med ends, one or both of which have central portions already polished, whereby the remaining annular end portions and sides can be polished using a small amount of liquid.

Another object of my invention is to provide an improved method for fabricating and chemically polishing or electropolishing the interior surface of a large cylindrical vessel having domed ends by rotating the vessel on its inclined axis whereby the forces against the axial thrust supports are minimized and the vessel is more easily supported.

Still another object of the invention is to provide simple readily fabricated apparatus for supporting a large cylindrical vessel with closed ends in an inclined position and rotating the vessel about its longitudinal axis during electropolishing or chemical polishing of its entire inner surface and thereby minimizing the quantity of polishing liquid required.

Certain other objects of the invention will, in part, be obvious and will in part appear hereinafter.

For a more complete understanding of the nature and scope of the invention, reference may be had to the following detailed description thereof taken in conjunction with the accompanying drawings wherein:

FIG. 1 is a vertical longitudinal sectional view of a large cylindrical vessel having substantially closed ends and shown supported for rotation about its inclined longitudinal axis with electropolishing elements suitably installed therein for use in accordance with the present invention;

FIG. 2 is a sectional view taken generally on line 2-2 of FIG. 1;

FIG. 3 is a sectional view taken on line 3-3 of FIG.

FIG. 4 is a vertical sectional view through one of the domed ends before being welded to the main cylindrical body and showing an electropolishing bath in position to polish the central portion;

FIG. 5 is a vertical longitudinal sectional view of a large cylindrical vessel having a centrally polished lower domed end, the vessel being inclined with an electropolishing bath in position to polish the remaining half of the lower inner surface; and

FIG. 6 is a fragmentary vertical sectional view of the domed end of a vessel in which the access nozzle is not axially located and showing a support for installing the electropolishing elements in an off-center nozzle.

Referring to FIGS. 1, 2 and 3, a large cylindrical vessel is indicated generally at having opposed substantially closed ends 7 and 8 welded to the opposite ends of the main cylindrical body 6. The substantially closed ends 7 and 8 have center protruding ports or nozzles 10 and 11, respectively. The vessel 5 is typical of large industrial vessels. The interior surface of the vessel 5 is polished in two steps in practicing the present invention. In the first step the vessel 5 is cradled for rotation about its longitudinal axis 32 by placement on spaced pairs of supporting rollers indicated generally at 12, 13 and 14. Each set of rollers 12, 13 and 14 consists of two sepa rate rollers 15-15, 16-16, and 17-17 mounted on supporting stands 18, 20 and 21, respectively, with the stub shafts supporting the rollers being parallel in each set. The supporting stands 18, 20 and 21 are of different dimensions and are located so as to support the vessel 5 so that its longitudinal axis 32 is inclined to the extent necessary to bring the center point X on the inner surface of the lower end 8 and the lowest point Y on the inner surface of the elongated cylindrical body portion 6 located halfway between ends 7 and 8 to approximately the same level. Supporting stand 18 is taller than supporting stand 20 while the respective rollers 15-15 and 16-16 are attached thereto so that their stud shafts are approximately parallel to the cylinder body 6. Supporting stand 17 is located to provide axial thrust support for the lower substantially closed end 8 with roller 17-17 aligned so that they make tangential contact wth end 8.

A pinion gear 22 is mounted on the end of the stud shaft supporting one of the rollers 16 and has meshing engagement with a pinion gear 23 carried on the drive shaft of an electric motor 24. it will be understood that when motor 24 is energized and drives the gear 23 the roller 16 will be driven and this will be sufficient to slowly rotate the vessel 5 about its longitudinal axis 32.

After the vessel 5 has been mounted on the inclined rollers 15, 16 and 17, a cathode assembly is inserted into the interior of the vessel. This assembly comprises a long metallic shaft or rod 25 which may be either solid or hollow, preferably the latter; one or more spider-like shaft supports generally at 26, which comprises a center bearing 27 made from insulating or dielectric material such as wood or plastic supported by a plurality of legs 44. A wire mesh cathode generally designated 28 is supported in a pendant manner from shaft 25. Cathode 28 is desirably fonned of copper screen and may be rolled, folded or disassembled for insertion through one of the protruding access nozzles or ports 10 or 11. The cathode 28 comprises a sector shaped end section 30 and an elongated section 31. Since the vessel 5 is symmetrical about its longitudinal axis 32 the cathode section 31 preferably will be shaped as a segment of a cylinder, with the width being less than the maximum dimension of the ports of nozzles 10 or 11. The end cathode member 30 is connected to the side cathode member 31 and is sector shaped with its apex angle being disposed at or near the shaft 25. When necessary to facilitate insertion and removal from the vessel, hinges 41 can be used to connect the end cathode member 30 to side cathode member 31. The end section 30 is contoured to conform to the curvature of the inner surface of the vessel ends 7 and 8. Insulating rollers 33 may be secured to the outer surface of the cathode 28 to provide uniform spacing between the cathode and the interior of vessel 5.

ln order both to prevent slippage at block 40 and to provide pendant support and supply of electrical current to cathode 28, shaft 25 is provided with a plurality of grooves 34-34 (three being shown). It will be appreciated that the grooves can in effect be provided by securing sets of spaced collars on rod 25. When the shaft or rod 25 is tubular or hollow these sections 34 may be conveniently provided by short solid pins of appropriate diameter telescoped within opposing ends of the tubular or hollow sections. The side cathode member 31 is supported in a pendant manner from the shaft or rod 25 by one or more V-shaped supports or hangers 36. Each of the supports 36 is in the form of a stiff metal strap or rod with the bite portion curved so as to hang freely in one of the grooves or recesses 34. The end cathode member 30 is brazed or otherwise connected to an electrically conductive collar 35, which rides in one of the grooves or recesses 34.

It will be seen that all of the elements of the cathode assembly may be readily inserted through one of the access ports or noules 10 or 11 into the vessel 5 and appropriately assembled therein. After the cathode assembly is installed within the vessel 5, electropolishing solution or bath is introduced into the vessel 5 until the surface 37 of said bath is slightly above both the center point X of the lower end and the lowest point Y on the portion of the body 6 located halfway between ends 7 and 8. With the rollers 15, 16 or 17 formed of metal, and the stub shafts and mountings for the rollers also being formed of metal, adequate grounding for the rotating metallic vessel 5 is provided so that the current may be passed through the solution when the cathode is appropriately connected as by connector or conductor 37 to the appropriate terminal or pole of a source of direct current. Altemately, instead of grounding through the rollers and supports, one end of a flexible cable may be clamped to the vessel and the other end grounded to a DC source.

Wooden blocks 38 and 40 are bolted to the outer flanges of the access ports or nozzles 1 1 and 10, respectively, these blocks having central holes in which shaft 25 is journaled, the central hole being small enough to provide a fairly fluid tight seal for the stationary shaft 25. The wooden block 38 at the lower end 8 is tightly drawn up to make a fluid tight seal while the wooden block 40 at the higher end 7 is only loosely held in place so that adequate venting of vessel 5 is provided. Alternately, a separate vent hole or holes may be drilled in block 40.

With the cathode apparatus and polishing solution in place as described above and electricity flowing from the vessel interior to the cathode 28, the vessel 5 is rotated intermittently or continuously in the inclined position and the lower half or portion of the inner surface is electropolished until the desired finish is obtained. The cathode apparatus and polishing solution are then removed and vessel 5 is then disposed so that the electroprocessed end is the higher end and the longitudinal axis is inclined as previously described. The cathode apparatus is installed as before in the lower unpolished end of the vessel 5 and the electroprocessing solution or bath is introduced as before and the lower half of the inner surface is electroprocessed to the desired finish.

One skilled in the art will understand that a cathode apparatus of the type with opposite triangular shaped ends, as disclosed in my US. Pat. No. 2,861,937, supported on bar 25 can also be used. If such a cathode apparatus extending the full length of a vessel is used the spider-like support 26 may be omitted if the vessel is not too long. Using such apparatus it will not be necessary to remove the cathode apparatus and bath prior to the step of placing the first polished end in the higher position.

Referring now to FIGS. 4 and 5, a method is illustrated therein for fabricating and chemically polishing or electropolishing substantially the entire inner surface of large cylindrical vessel generally at 45 with domed ends 46 and 47, using a very small amount of polishing liquid 48 and 50.

In FIG. 4, before the lower domed end 46 is welded to the cylindrical body, a blind flange 51 is attached to make a liquid tight seal with nozzle 52. End 46 is then placed with flange 51 down and horizontal. A small amount of polishing bath 48 is added to reach an intermediate level at point Z. The central portion of end 46 is the polished to the desired finish using one of the methods for polishing domed ends taught in my US. Pat. No. 3,682,799, dated Aug. 8, 1972, the disclosure of which is incorporated by reference herein.

Referring to FIG. 5, after the central portion of end 46 has been polished, it is circumferentially butt welded at 53 to cylinder body 54 in a known manner. The cylindrical vessel 45 is then cradled for rotation about its inclined longitudinal axis 55 by placement of spaced pairs of supporting rollers 56, 57 and 58 of the type previously described in connection with FIG. 1. The supporting stands 60, 61 and 62 are of different dimensions and are located to provide a support vessel 45 so that its longitudinal axis 55 is inclined only to the extent necessary to bring the lowest point Z on the edge of the previously polished central portion of lower end 46 and the lowest point W on the inner surface of the cylinder body 54 located halfway between ends 46 and 47 to approximately the same level.

A cathode assembly 64 is inserted into the interior of the vessel; this assembly 64 will be similar to that previously described in connection with FIGS. 13. After this assembly 64 is installed within vessel 45, the polishing liquid 50 is introduced into the vessel 45 until the surface 63 of said bath is slightly above both the lowest point Z on the edge of the previously polished central portion of lower end 46 and the point W on the inner surface of the cylinder body 54 located halfway between ends 46 and 47. The vessel 45 is then suitably grounded and vented as previously described. Polishing of the remaining unpolished inner surface of the lower half of vessel 45 is then achieved by passing electricity from the vessel interior to the cathode while rotating vessel 45 about its inclined longitudinal axis 55.

It will be understood by one skilled in the art that when it is desired to polish both domed ends 46 and 47 by this method, then the central portion of end 47 can be polished as described above and shown in FIG. 4, prior to being welded to cylinder body 54. In that instance, after the first half of the inner surface of vessel 45 is polished, the cathode apparatus and polishing liquid are removed and vessel 45 is disposed so that the completely polished end is the higher end and the longitudinal axis is inclined as previously described. The

cathode apparatus is installed as before in the lower unpolished portion of vessel 45, and the polishing solution is introduced as before. The remaining unpolished inner surface of the lower half of vessel 45 is then polished to the desired finish.

One skilled in the art will understand that my method is not dependent on a precise initial location for points 2 and W. Rather, they may be located throughout a range of possibilities, so long as Z is a point on the domed end and W is a point on side wall. When points Z and W are not precisely located, sufficient liquid must be introduced in the first polishing step in the inclined position to raise the surface 63 of the polishing liquid above the lowest point Z on the edge of the previously polished central portion of the lower end 46. Then, in the second polishing step in the inclined position, sufficient liquid must be introduced to wet, when it is rotated, all of the remaining unpolished inner surface of vessel 45.

Referring now to FIG. 6 an arrangement is shown for supporting shaft 71 when the access nozzle 72 of the domed end is not axially located but off-center. Blind flange 73 is bolted to nozzle 72 to fonn a liquid tight seal. Shaft end receiving socket 74 issupported from the inside of flange 73 by support arm 75. The socket 74 is positioned on the longitudinal axis of the vessel 77 and close to the domed end 70. The cathode support shaft 25 is then inserted in socket 74 and the method is practical as previously described. In this modification it will be apparent that if insulating rollers for the cathode are desired (33 in FIG. 1) they will be located to contact that portion of domed end 70 radially further from the longitudinal axis than nozzle 72.

It will also be understood that alternative methods of achieving the inclined longitudinal axis may be used. For example the vessel may be appropriately inclined by using only one set of roller supports at one end as 5757 in FIG. 5, and by raising and supporting the other end of the vessel with a spool or well greased or lubricated sling attached to an overhead crane.

While the previous description disclosed a method for electroprocessing, one skilled in the art will also understand that it will work equally well for chemical polishing. In that instance the polishing bath will be one wherein the polishing action occurs chemically and the use of a cathode is not required. For example, vessels 5 and 45 may be formed of aluminum and the polishing bath may have a composition as disclosed in my US. Pat. Nos. 2,705,191 and 3,041,227.

One skilled in the art will also understand that the method I have described will work equally well for any type of processing of the inner surface wherein a bath of processing liquid is introduced and the vessel is rotated about its longitudinal axis during the processing. It will further be understood that the method I have described will work equally well with most vessels which are concentric about their longitudinal axis, examples of such vessels are cone-shaped vessels, funnel-shaped vessels, or others.

While I have described my method in the context of polishing the inner surface of a vessel with centered openings at each end, one skilled in the art will understand that my method will also work equally well with vessels which have only one end closed, in which case any electrode apparatus used during polishing can be inserted from the open end and supported by suitably located spider-like supports. Likewise, when it is desired to polish one end and only a portion of the remaining inner surface of the vessel, whether the other end is open or closed, it will be understood that my method will work equally well.

One skilled in the art will also understand that my method will work equally well when both ends of the vessel are closed but only one end of the vessel has an access port and that access port is not located on the longitudinal axis; for then the apparatus of the type described in my US. Pat. No. 2,861,937 can be used.

One skilled in the art will further understand that while I have described the method as polishing the entire inner surface of the vessel approximately one-half at a time; in those situations wherein minimizing the volume of the polishing liquid is not critical or important, one end and more or less than one-half of the body may be polished in the first polishing step, and the remaining end and that portion of the unpolished body can be polished in the second polishing step.

One skilled in the art will further understand that neither the polishing nor the rotation need be continuous, and that my method will work equally well when they are discontinuous. For example the rotation may be step-wise or intermittent and the method will work equally well.

It will be apparent that in practicing my invention all of the polishing bath is confined within the vessel, the interior of which is being polished, and the volume of the bath will be substantially less than one-half the full capacity of the vessel.

I claim:

1. In the method of polishing the interior of an elongated vessel substantially symmetrical about its longitudinal axis and containing a bath of polishing liquid selected from the group consisting of an electropolishing bath and a chemical polishing bath while the vessel is rotated about its longitudinal axis, the improvement which comprises, supporting said vessel so that said longitudinal axis is materially inclined with respect to the horizontal during rotation and simultaneously polishing substantial portions of the inner surfaces of the lower end and side wall of said vessel while limiting the upper level of said bath so as not to cover more than about the lower half of the inner surface of said lower end, said bath being confined entirely within said vessel and having a volume substantially less than one-half the capacity of said vessel.

2. The improvement called for in claim 1 wherein approximately one-half of the inner surface of said side wall is polished while at least a substantial portion of the inner surface of said lower end is being polished.

3. The improvement called for in claim 1 wherein the entire inner surface of said lower end and approximately one-half of the inner surface of said side walls are simultaneously polished.

4. The improvement called for in claim 1 wherein the rotation is continuous.

5. The improvement called for in claim 1 wherein the rotation is discontinuous.

6. In the method of polishing the interior of an elongated vessel substantially symmetrical about its longitudinal axis and containing a bath of polishing liquid selected from the group consisting of an electropolishing bath and a chemical polishing bath while the vessel is rotated about its longitudinal axis, the improvement which comprises, supporting said vessel so that said longitudinal axis is inclined sufficiently to allow said bath to wet approximately one-half of the inner surface of the side wall and at least a substantial portion of the inner surface of the lower end of said vessel during rotation while limiting the upper level of said bath so as not to cover more than about the lower half of the inner surface of said lower end, and reversing the position of said vessel so as to simultaneously polish the remaining unpolished portion of the inner surface of said side wall and at least a substantial portion of the remaining unpolished end of said vessel, said bath being confined entirely within said vessel and having a volume substantially less than one-half the capacity of said vessel.

7. The improvement called for in claim 6 wherein the entire inner surfaces of the opposite ends of said vessel are polished during the polishing of the inner surface of said side wall thereof.

8. The improvement called for in claim 6 wherein first a central portion of each end of said vessel is polished prior to said ends being welded to the elongated body.

9. The improvement called for in claim 6 wherein the rotation is continuous.

10. The improvement called for in claim 6 wherein the rotation is discontinuous. 

1. IN THE METHOD OF POLISHING THE INTERIOR OF AN ELONGATED VESSEL SUBSTANTIALLY SYMMETRICAL ABOUT ITS LONGITUDINAL AXIS ANDI CONTAINING A BATH OF POLISHING LIQUID SELECTED FROM THE GROUP CONSISTING OF AN ELECTROPOLISHING BATH AND A CHEMICAL POLISHING BATH WHILE THE VESSEL IS ROTATED ABOUT ITS LONGITUDINAL AXIS, THE IMPROVEMENT WHICH COMPRISES, SUPPORTING SAID VESSEL SO THAT SAID LONGITUDINAL AXIS IS MATERIALLY INCLINED WITH RESPECT TO THE HORIZONTAL DURING ROTATION AND SIMULTANEOUSLY POLISHING SUBSTANTIAL PORTIONS OF THE INNER SURFACES OF THE LOWER END AND SIDE WALL OF SAID VESSEL WHILE LIMITING THE UPPER LEVEL OF SAID BATH SO AS NOT TO COVER MORE THAN ABOUT THE LOWER HALF OF THE INNER SURFACE OF SAID LOWER END, SAID BATH BEING CONFINED ENTIRELY WITHIN SAID VESSEL AND HAVING A VOLUME SUBSTANTIALLY LESS THA ONE-HALF THE CAPACITY OF SAID VESSEL.
 2. The improvement called for in claim 1 wherein approximately one-half of the inner surface of said side wall is polished while at least a substantial portion of the inner surface of said lower end is being polished.
 3. The improvement called for in claim 1 wherein the entire inner surface of said lower end and approximately one-half of the inner surface of said side walls are simultaneously polished.
 4. The improvement called for in claim 1 wherein the rotation is continuous.
 5. The improvement called for in claim 1 wherein the rotation is discontinuous.
 6. In the method of polishing the interior of an elongated vessel substantially symmetrical about its longitudinal axis and containing a bath of polishing liquid selected from the group consisting of an electropolishing bath and a chemical polishing bath while the vessel is rotated about its longitudinal axis, the improvement which comprises, supporting said vessel so that said longitudinal axis is inclined sufficiently to allow said bath to wet approximately one-half of the inner surface of the side wall and at least a substantial portion of the inner surface of the lower end of said vessel during rotation while limiting the upper level of said bath so as not to cover more than about the lower half of the inner surface of said lower end, and reversing the position of said vessel so as to simultaneously polish the remaining unpolished portion of the inner surface of said side wall and at least a substantial portion of the remaining unpolished end of said vessel, said bath being confined entirely within said vessel and having a volume substantially less than one-half the capacity of said vessel.
 7. The improvement called for in claim 6 wherein the entire inner surfaces of the opposite ends of said vessel are polished during the polishing of the inner surface of said side wall thereof.
 8. The improvement called for in claim 6 wherein first a central portion of each end of said vessel is polished prior to said ends being welded to the elongated body.
 9. The improvement called for in claim 6 wherein the rotation is continuous.
 10. The improvement called for in claim 6 wherein the rotation is discontinuous. 